The atomic number of strontium is 38 according to the Element Tables. The atomic number of plutonium is 94. So the second nucleus on the right must have atomic number 56 (94 - 38).
The Element Tables show that element number 56 is barium (Ba).
We subtract mass numbers to find the number of free neutrons. We have 240 on the left, and 90 + 142 = 232 on the right. The difference is 8. There are 8 free neutrons.
In the isotope is 142Ba, there are 86 neutrons (86 = 142 - 56). The number of neutrons in the nuclei are:
A total of 138 neutrons on the right (52 + 86), 146 neutrons in the plutonium, a net of 8 neutrons left over, in agreement with the first calculation.
The equation of the fission process is: